There are many similarities in structure and properties between the human hepatitis B virus (HBV) and the duck hepatitis B virus (DHBV). These similarities suggest that DHBV infection in ducks may be a useful experimental model of human HBV infection, particularly as HBV cannot be grown readily in cell culture. Ducks infected with DHBV at birth become chronic carriers of the virus, although they may not develop overt hepatitis. Some DHBV-infected ducks have been reported to develop hepatocellular carcinoma, a tumor strongly linked etiologically in humans with chronic hepatitis B infection. As a screening test for new effective therapies for chronic type B hepatitis in man, new antiviral immunomodulatory agents are being assessed for their ability to suppress DHBV replication in ducks.